The only concern I would have is the rad itself... I know it isn't as much of an issue with fresh water as with salt, but I remember that when I was growing up we had freshwater fish, and there was still a big deal about not putting metal in the tank.
I suspect that if you were to stick either a Cu/Brass or Al rad in your tank, you might have problems with it corroding (That water isn't pure, the dang fish keep pissing in it!
) and the corrosion products killing off the fish.
I think it would work from the cooling end, but I would suggest running the idea by some aquarium experts to be sure it's OK from the fish health end...
Gooserider